WO90/08970 describes a procedure for poling an optical glass fiber by applying a transverse, high electric field at elevated temperatures. The poling imparts permanent second order nonlinearity to the fiber. A transverse electric field applied to the poled fiber induces refractive index changes proportional to the field strength (Pockels effect). In contrast, unpoled fibers (having macroscopic inversion symmetry) exhibit the Kerr effect (i.e. the index change is very small and varies in proportion to the square of the field strength). WO90/08970 and WO 97/01100 describe voltage sensors using a poled fiber. The fiber describes helical or spiral-like paths running from ground to high voltage potential. The light waves in the fiber experience an optical phase shift which is a measure for the voltage. The phase shift is measured in a Mach-Zehnder interferometer or by polarimetric means.
However, this type of sensor can be sensitive to various external parameters, such as temperature, mechanical shock and vibration, which can lead to an optical phase change and can therefore seriously deteriorate the voltage measurement.
WO 03/023320 discloses an electro-optic voltage sensor having a poled optical fiber in which the poling direction is reversed and a 90°-splice swaps the polarization modes at mid-length. Two orthogonal polarization modes are injected by using a Faraday rotator with 90° rotation angle per pass.